Description

The Tomo statics 3D module derives near-surface static corrections for 3D surveys by performing refraction tomographic inversion of first-break travel-time picks. The module iteratively builds a 3D near-surface velocity model and computes source and receiver static corrections relative to a reference datum. The inversion uses a fast-marching eikonal solver to compute travel-time tables and applies a regression-based pick interpolation scheme to stabilise the solution in areas with variable pick coverage.

This module is an advanced 3D-dedicated implementation designed for large 3D land surveys with complex near-surface conditions. It accepts a BinGrid to define the output model domain and supports optional GPU acceleration for the travel-time computation step. This module is deprecated; new projects should use the current Tomo statics 2D/3D module.

Input data

Output geometry

Link to a BinGrid item defining the spatial extent and inline/crossline numbering of the 3D survey. The module uses this grid to define the tomographic model domain and to assign output velocity values to the correct 3D spatial locations.

Input trace headers

Link to the trace header vector containing source and receiver coordinates for all traces. The tomographic ray tracer uses these positions to set up the source-receiver geometry for each pick.

Input SEG-Y data handle

Link to the SEG-Y file handle providing the seismic data from which first-break picks are read or displayed for quality control during the inversion.

Parameters

Velocity model params

Container group controlling the parameterization of the near-surface velocity model.

Deepest refractor velocity

Velocity (m/s) of the deepest refractor layer used as the upper velocity bound in the initial near-surface model. Default: 2500 m/s. Set this to the expected velocity at the base of the weathering zone.

Datum

Reference datum elevation (m) to which static corrections are reduced. Default: 1000 m. Set this to the floating or final datum used in your processing flow.

Max depth

Maximum depth (m) of the near-surface velocity model. Default: 800 m. The model is built from the surface down to this depth. Set this to the maximum depth illuminated by the longest usable first-break offsets.

Horizontal smooth

Lateral smoothing radius (m) applied to the velocity model update at each iteration. Default: 100 m. Increase this value to produce a more geologically plausible smooth model when pick coverage is sparse.

Frequency

Dominant frequency (Hz) of the first-break arrivals. Default: 10 Hz. Used to compute the Fresnel zone width for weighting ray contributions to the velocity update.

Fast

When enabled, uses a computationally efficient approximation for the travel-time calculation. Default: enabled. Disable for higher-accuracy travel-time computation at the cost of longer run times.

Number of velocity updates

Total number of outer inversion iterations. Default: 8. Each iteration re-traces all rays through the updated velocity model. Increasing this value improves convergence but proportionally increases computation time.

Enable velocity inversion

When enabled, permits the inverted model to contain low-velocity zones. Default: disabled. Enable only when geological evidence supports the existence of a velocity reversal.

Picking interpolation

Method used to interpolate first-break picks onto the model grid where picks are absent. Options include nearest-neighbour, regression-based, and triangulation methods. Choose a regression-based method for noisy data and nearest-neighbour for dense, high-quality picks.

Velocity min

Minimum velocity (m/s) permitted in the inverted model. Default: 500 m/s.

Velocity max

Maximum velocity (m/s) permitted in the inverted model. Default: 7000 m/s.

Offset min

Minimum source-receiver offset (m) of picks used in the inversion. Default: 0 m.

Offset max

Maximum source-receiver offset (m) of picks used in the inversion. Default: 3000 m. Exclude far-offset picks where first-break signal quality deteriorates.

Offset step

Offset bin width (m) used to group picks by offset range during the inversion. Default: 500 m. Larger offset steps reduce computation time but decrease the resolution of offset-dependent velocity constraints.

Maximum deviation (picking to model)

Maximum acceptable difference (m) between an observed first-break pick and the modelled travel time. Picks exceeding this threshold are treated as outliers and excluded. Default: 300 m.

TopoFilling

Container group controlling how the model is filled above the acquisition surface in areas where topography rises above the datum.

Fill topo

When enabled, fills model cells above the acquisition surface with a constant replacement velocity defined by the Velocity filling parameter. Enable this when the topography intersects the model volume and empty cells would otherwise cause artefacts in the static corrections.

Velocity filling

Replacement velocity (m/s) used to fill model cells above the acquisition surface when Fill topo is enabled. Set this to the replacement velocity of the medium above the surface (typically air velocity or a nominal low value).

Samples topo fill

Number of model samples filled above the acquisition surface with the replacement velocity. Increase this when the acquisition surface has high relief relative to the model grid spacing.

Solver params

Container group with parameters controlling the tomographic solver iterations and the residual statics estimation.

Calculate residual

When enabled, computes residual static corrections after the tomographic velocity update to account for small pick errors and near-surface anomalies not resolved by the velocity model. Default: enabled.

Residual vel interpolation

Interpolation method used to distribute residual velocity corrections between the control points. Options include 4D bilinear, spatial regression, and nearest-neighbour methods.

Regression aperture

Lateral aperture (m) of the local regression used to estimate the velocity update from nearby first-break picks. Default: 420 m. Use a larger value in areas with low pick density to include more picks per regression window.

Max pick time deviation

Maximum permissible travel-time residual (s) between a first-break pick and the modelled travel time. Picks with residuals above this value are excluded from the residual statics computation. Default: 0.05 s (50 ms).

Global iterations count

Number of global solver iterations in the residual statics step. Default: 4.

Local iterations count

Number of local iterations in the residual statics solver. Default: 4.

Regression factor

Weighting factor controlling the contribution of the regression estimate to the velocity update relative to the direct tomographic update. Higher values increase the influence of the smoothed regression estimate.

Maximum picks regression

Maximum number of picks included in each local regression window. Default: 5000. Limiting this number keeps each regression computation tractable on large datasets.

Calculation area

Container group restricting the tomographic computation to a sub-region of the 3D survey defined by inline and crossline number ranges. All parameters default to -1 (no limit), which means the full survey extent is used. Restrict the computation area to speed up testing on a specific sub-region of the survey.

First inline number(-1 no limit)

First inline number of the active calculation area. Set to -1 to use the survey minimum.

Last inline number(-1 no limit)

Last inline number of the active calculation area. Set to -1 to use the survey maximum.

First crossline number(-1 no limit)

First crossline number of the active calculation area. Set to -1 to use the survey minimum.

Last crossline number(-1 no limit)

Last crossline number of the active calculation area. Set to -1 to use the survey maximum.

Step X depth out

Inline spatial sampling (m) of the tomographic velocity model. Default: 600 m.

Step Y depth out

Crossline spatial sampling (m) of the tomographic velocity model. Default: 600 m.

Step Z depth out

Vertical sample interval (m) of the tomographic velocity model. Default: 10 m.

Step X depth time table

Inline grid spacing (m) of the internal ray-tracing travel-time table. Default: 50 m.

Step Y depth time table

Crossline grid spacing (m) of the internal travel-time table. Default: 50 m.

Step Z depth time table

Vertical sample interval (m) of the internal travel-time table. Default: 10 m. Finer spacing improves accuracy but increases memory consumption.

Compress factor time table

Integer compression factor applied to the vertical axis of the travel-time table to reduce memory usage. Default: 1 (no compression). A value of 2 halves the number of depth levels stored in the table. Increase this when available RAM is insufficient to hold the full travel-time volume.

Custom actions

Solve

Launches the full 3D tomographic inversion using the loaded first-break picks. The module runs all inversion iterations, outputs the near-surface velocity model, and computes source and receiver static corrections.

Import FBPicking

Imports first-break pick times from an external ASCII or project file into the module for use in the inversion. Use this to load picks created in a separate picking session or in a third-party application.